Hot to Nuetral question

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dav5y

Member
Location
NYC
In my previous life I was an electrician, electrical engineer now, but got stumped on a situation I ran into today doing a side job. Was tying to find a feed for a light fixture (residential job) and was getting approx. 60v from hot to neutral and ground from the nearest splice box... Ran a new line for it to work , but could not figure it out.... I'm thinking a bad neutral ???????
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
In my previous life I was an electrician, electrical engineer now, but got stumped on a situation I ran into today doing a side job. Was tying to find a feed for a light fixture (residential job) and was getting approx. 60v from hot to neutral and ground from the nearest splice box... Ran a new line for it to work , but could not figure it out.... I'm thinking a bad neutral ???????

You may have been testing a switch leg that was off or phantom voltage.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
As a EE, consider what voltage you would read with a high impedance meter when measuring an insulated open conductor which runs many feet at an equal (short) distance between a hot wire and a grounded wire or in contact with both.


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GoldDigger

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It was not a bad question to ask and many an electrician has been perplexed by that the first time it happens. The good ones recognize it the second time. Some take longer.
It drives home that for some purposes the "better" voltmeter with an input impedance in the megohm range and with three digit accuracy may not be as helpful as a solenoid type tester. :)

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dav5y

Member
Location
NYC
Hi GoldDigger,

Actually, I ran into that problem once testing an ungrounded delta transformer for a power plant.... what happened was the original bus duct fused two phases together (water damage)...... if that makes sense.... and formed a "ground". The new VFD w/ GFI protection for Chillers would not work... Had to run new 2000a (480V) bus duct bus duct for it to work.....

Andrew
 

kwired

Electron manager
Location
NE Nebraska
Capacitive coupling is the key term. The document in the link Don posted does explain it pretty well. A test meter with low impedance draws enough current to discharge the "capacitor" faster than it sustain, regain, replenish... That voltage is real, but the amount of power behind it is so low it is not a touch hazard, touching it often shorts out the capacitor also, but unless you know for certain it is capacitive coupled voltage probably is not a good idea to touch.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Sorry will not happen again. Was just curious....
Nothing to be sorry or embarrassed about. Every sparky scratches their head the first time they encounter it. As it turns out this is one case where technology (high impedance DVM) are not an improvement.

If it makes you feel better here is my little story going on today. I have been in electrical for close to 40 years now. I started in high school vo-tech and got my license, put myself through school obtained my engineering degree. Anyway going through a home remodel project and tracing circuits back to breakers. Bought myself one of those touch less testers. Did not take long to learn not to trust them. So ended up making a test lamp to plug in to confirm power was on/off. Nothing more than a small table lamp and 25 watt light bulb. Works every time. Touch Less Tester not so much. :ashamed1:
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
Interesting quote below from the link;

It?s not uncommon practice for electrical contractors to think ahead for their customers and pull extra circuits for possible future expansion of the electrical
system.
Maybe the Copper Development Association helped Fluke write that document.:)
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
Nothing to be sorry or embarrassed about. Every sparky scratches their head the first time they encounter it. As it turns out this is one case where technology (high impedance DVM) are not an improvement.

I have a story that should make everybody feel better about encountering this issue. I use a Fluke digital volt meter, btw.

One time I went to an old school building to fix a circuit not working. Someone had added a bathroom adjacent a second story office, and now all of a sudden one of the office plugs doesn't work. Surface mounted handy box, wired in mc that disappears into a plaster wall. Fluke tells me it has about 46 volts. Oh, ok. Just find the loose neutral. I just need to get behind that wall where the wire disappears. Not easy at all.

I find an access that takes me between the ceiling and floor above, make my way over to that area, and find a plumbing chase right where I need to be looking. Shine my flashlight down there and see a 1900 box with about three mc cables in it, one which goes through the plaster. Also see a couple of Romex cables in it going out to the bathroom light and plug. AHA. Some "Tommy Two-Twist" didn't pre-twist the wires when he landed those two Romex cables.

So I come out of the ceiling, go back to the office, and tear a hole in the plaster wall to get to that box. I'm laying on my stomach to reach far enough into the hole to get into to that box. By this time, I've got about three hours in it and haven't even opened the 1900 box yet.

So I struggle to get into that box. Take the lid off, and find three circuits in it. It was packed. Well, I'd better go turn off the breakers so that I can tear everything apart and make it up right.

I go to the panel and find that there's a breaker tripped. HUH ?!? Wonder what that goes to? Panel schedule says it goes to second floor office plug. HUH ?!?

My heart sank. Is that even possible for a tripped breaker to do that? No way, man. I'd never heard of such a thing. Man, if that fixes it, I'm gonna feel really stupid.

So I reset the breaker and go back up and test the plug. 123 volts. Ouch. Let me go turn the breaker off and check it again. Now I've got 46 volts again. Turn breaker on again, 123 volts again. Ouch !

Yeah, I felt pretty stupid alright. Now my next dilemma - explaining to my boss how I had just spent 4 hours to reset a tripped breaker, and ended up with a hole in a plaster wall. Dang it.

Went home and did some research after that. I will never forget it as long as I live
 

Clayton79

Member
Location
illinois
Occupation
Owner/operator
How about this one, if I remember correctly, it happened maybe a decade ago. I was replacing an outlet in a bedroom, shut off breaker feeding it and had yup 60volts, I un hooked the wires from the breaker box and checked the breaker to ground/neutral buss with the breaker off and still had a reading of 60vac this was on a fluke 87 I believe, I replaced the breaker with a new one and no longer read any voltage in the off position, I just assumed ( I know:happyno:) that is was a bad breaker. I would love to hear some more ideas on about what might have happened.
 

kwired

Electron manager
Location
NE Nebraska
How about this one, if I remember correctly, it happened maybe a decade ago. I was replacing an outlet in a bedroom, shut off breaker feeding it and had yup 60volts, I un hooked the wires from the breaker box and checked the breaker to ground/neutral buss with the breaker off and still had a reading of 60vac this was on a fluke 87 I believe, I replaced the breaker with a new one and no longer read any voltage in the off position, I just assumed ( I know:happyno:) that is was a bad breaker. I would love to hear some more ideas on about what might have happened.
Likely foreign material ended up inside the breaker bridging the open circuit gap. It should have still read 120 volts though with no load. Maybe the use of a low impedance meter would have registered no voltage, as it would load the circuit enough to drop the voltage to nothing if there were a high impedance bridging the open breaker.
 

rlundsrud

Senior Member
Location
chicago, il, USA
Very informative, I always assumed that this condition was caused by an induced voltage. I presumed that the changing magnetic field caused the open wire to act like the secondary of a transformer. I had never considered capacitive coupling as the cause.

Bob
 

hurk27

Senior Member
Very informative, I always assumed that this condition was caused by an induced voltage. I presumed that the changing magnetic field caused the open wire to act like the secondary of a transformer. I had never considered capacitive coupling as the cause.

Bob

With inductive coupling you would have to have a current flowing in the primary circuit, in some cases this may be true, but in many phantom voltages cases the live circuit may not have any current in it at all, and you can still read a voltage with a 10 meg ohm per volt meter or higher, I used to take an old extension cord one with the ground prong missing and show my workers how a high impedance DVM will show a small voltage on that EGC in the cord even though it has no connection to anything, and there is no load on the cord while doing this, it was a 150' cord that I eventually repaired but it did teach a few about phantom voltages and what to look out for.
 
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